The present invention relates to a technology of digital watermarking for binary images which is designed to embed arbitrary information in binary images and detect it.
Digital watermarking for binary images is a technology of inverting the luminance value of some pixels at specific positions where modification does not stand out so conspicuous in the digital data of binary images such as documents, tables, and maps, thereby embedding information such as copyright ID in them to protect the copyright of binary images.
The method for embedding information in binary images should meet the following requirements from the practical point of view.
(1) It should be applicable to not only documents consisting solely of characters but also general binary images.
(2) It should be able to embed digital watermarks without being perceived unless watermarking consists of a large amount of information. Alternatively, it should be able to embed digital watermarks without interfering with the recognition of images even though they are perceivable.
(3) It should produce the digital watermarks (embedded information) that can be normally detected from printed paper as well as any images which have been enlarged or reduced.
(4) It should be able to control the amount of information to be embedded.
Among technologies of watermarking for binary images consisting solely of characters, there is known one which is designed to conceal information by subtly changing the character space or line space. See the document 1, S. H. Low, N. F. Maxemchuk, J. Brassil, and L. O'Gorman: “Document Marking and Identification Using Both Line and Word Shifting”, INFOCOM vol. 2, pp. 853-860 (1995), for example.
Among conventional watermarking technologies applicable to general binary images, there is known one which is designed to write a special pattern (corresponding to watermarking information) directly in the blank space of background. See the document 2, M. Bern, J. Breidenbach, and D. Goldberg: “Trustworthy Paper Documents”, LNCS 2147, pp. 1-12 (2001), for example. Moreover, the document 3 is known, which discloses another method of inserting watermarking information by making minute changes only in the edge of an image where modification is less conspicuous than that in the flat part of an image, N. Kobori, M. Iwakiri, and K. Matsui: “One technology for digital watermarking by pixel distribution applicable to binary cartoons”, Journal of the Information Processing Society of Japan, vol. 42, No. 3, pp. 596-598 (2001).